1. Field of the Invention
The present invention relates to removers for selectively removing anodized films formed as a result of anodization of aluminum or aluminum-alloy members. It also relates to methods for removing anodized films using the removers.
2. Description of the Related Art
Some aluminum or aluminum-alloy members are anodized in order to harden their surfaces or to impart corrosion resistance to the surfaces. When anodized films formed by anodization must be partially removed or be retreated, they are removed typically by chemical etching or shot blasting.
Examples of removers (etchants) used in chemical etching are (1) a mixture of phosphoric acid and chromic acid, (2) an aqueous sodium hydroxide solution, (3) a mixture of sulfuric acid and hydrofluoric acid, (4) a mixture of sulfuric acid and potassium fluoride, and (5) a mixture of nitric acid and hydrofluoric acid (“ARUMINIUMU HYAKKAJITEN (Encyclopedia of Aluminum)”, edited by KEIKINZOKU KYOKAI (Japanese Association of Light Metals)). Japanese Unexamined Patent Application Publication (JP-A) No. 2004-211128 discloses a method for removing oxide films by etching with a phosphoric acid/chromic acid solution, a sodium hydroxide solution, and/or a potassium hydroxide solution in a method for recycling aluminum parts for semiconductor equipment. JP-A No. 61-90777 discloses a method for removing anodized aluminum films not by a chemical process but by shot blasting, in consideration that conventional sulfuric acid treatment solutions corrode or are harmful to underlying metals.
The mixture of phosphoric acid and chromic acid must be kept at high temperatures of 95° C. to 100° C. for efficiently dissolving anodized films, and it requires much efforts and facilities to treat the waste liquid and effluent thereof, because the mixture contains environmentally harmful chromium, although the mixture does not damage aluminum or aluminum-alloy members as underlying metals. The aqueous sodium hydroxide solution dissolves underlying aluminum or aluminum alloys, which causes significant dimensional changes of members upon removal of anodized films, although the solution can efficiently dissolve and remove anodized films at temperatures of around room temperature to about 60° C. The mixtures of sulfuric acid with hydrofluoric acid, of sulfuric acid with potassium fluoride, and of nitric acid with hydrofluoric acid dissolve underlying aluminum or aluminum alloys, which causes significant dimensional changes of members upon removal of the anodized films as in the aqueous sodium hydroxide solution, although they can efficiently dissolve and remove anodized films at around room temperature.
Such methods for removing anodized films in which underlying aluminum or aluminum alloys are dissolved are not desirable for removing anodized films of members which require high dimensional accuracy as in semiconductor equipment. Mechanical methods for removing anodized films, such as shot blasting, cannot be applied to members having complicated shapes, although they can be applied to members having simple shapes, such as plates and rods.